If the Roman Emperor Constantine had not taken that critical decision in the first half of the 4th century, the picture of the world we live in would have been probably very different than today. As some historians speculate, we would have seen “Mithraeums” instead of churches (and probably mosques) throughout the western world (and the Middle East), decorated with iconographies of a sun cult, along with the mysterious “Tauroctony” reliefs on the innermost walls of the temples.

When the “brain team” of the emperor came with a suggestion to establish a “one strong official religion” in order to reinforce the central authority throughout the Empire, the strongest candidate was “Mithraism” (or as it was called after a metamorphosis on Roman lands, “Sol Invictus Mithra”) which was very popular among soldiers, officers and most intellectuals.

The Emperor himself was a loyal believer of Mithraism but he chose to build the urged official religion on Christianity, probably in order to eliminate his political rivals among army generals who could attempt a military coup anytime or setup military uprisings at any region – and most of these “dangerous” men were at high degrees of Mithraic order. (Military “interventions” became a “political tradition” in Rome after Sulla’s coup in the first century BCE and after Augustus, many emperors faced a similar fate.) So, presumably not to force his rivals’ hands by organizing Mithraism as the official “state religion”, he chose Christianity instead, which seemed “harmless”, mostly disorganized and most important, easy to manipulate. (The Nicea Council appeared as a good example of “official manipulations” on a messianic cult derived from Judaism.) He also enjoyed the benefits of the similarities between Christianity and Mithraism during his efforts to make the new official religion widely accepted.

The article below is from The Archaeology Magazine – an online feature that summarizes the characteristics of Mithraism or “Sol Invictus Mithrae” with a lot of images.

Of the religions that expanded rapidly in the 1st-century Roman Empire, worship of Mithras was particularly popular among Roman soldiers, who spread his cult during their far-flung travels. But no written evidence from the Mithraists themselves survives, and the literary evidence we have is mostly by Christian detractors. Mithras’s temples, called Mithraea, are the best archaeological evidence of the god’s worship, and most of them featured a characteristic depiction of Mithras slaying a bull, a scene called the tauroctony. Sifting through this imperfect record, scholars have been able to conjecture about many aspects of this once widely practiced religion.

Greco-Roman religious scholar Luther Martin says that Mithraism remained de-centralized throughout the Empire. Its contemporary, Christianity, got its central administration from St. Paul, who derived it from Judaism. Both it and Mithraism “were…pretty much locally controlled affairs,” he says, though Christian communities did “come together as a coherent institution…after Constantine.”

Greco-Roman religious scholar Luther Martin says that Mithraism remained de-centralized throughout the Empire. Its contemporary, Christianity, got its central administration from St. Paul, who derived it from Judaism. Both it and Mithraism “were…pretty much locally controlled affairs,” he says, though Christian communities did “come together as a coherent institution…after Constantine.”

In later years, Christian commentators recognized similarities between Mithraic and Christian rites and were quick to condemn them. In Chapter 70 of Dialogue with Trypho, the 2nd-century Christian author Justin Martyr writes that Mithras’s worship in a cave and his “rock birth”–a frequent depiction of the god, emerging from a stone–is taken from Daniel 2:34 and Isaiah 33. The Mithraists “have no understanding” of these Scriptures, says Justin.

Jesus was not the only deity with whom Mithras shared similarities. In the later Roman Empire, Mithras blended in with another sun god, Sol Invictus, the “unconquered sun.” Both gods appeared in the Spanish provinces around the same time, according to Jaime Alvar, an ancient history professor at the Universidad Carlos III de Madrid. Some 1st-century votive offerings in Rome even conflate the two gods into one deity, “Sol Invictus Mithras.”

By the 5th century, Mithraism faded. However, Mithras and Sol Invictus have echoes in the worship of Jesus Christ. Martin believes the ideas of brotherhood in Mithraism and apostleship in Christianity descend from collegia, or Greek social and political clubs. “My own take is that you’ve got two religions developing at the same time and in the same place and in the same culture and they’re going to develop similar kinds of expressions, symbolic expressions,” he adds.